TUESDAY, July 17 (HealthDay News) -- Interferon beta, a widely used treatment for multiple sclerosis, does not stave off the time to disability, new research finds.

However, prior studies have found that interferon beta does reduce MS flare ups, so patients should continue taking it, researchers said.

The new study is published in the July 18 issue of the Journal of the American Medical Association.

In multiple sclerosis, the body's immune system attacks myelin, or the substance that insulates nerve fibers of the central nervous system. The damage disrupts nerve signals traveling to and from the brain, which can lead to numbness, movement difficulties, blurred vision, fatigue and eventually, problems with thinking and memory.

About 85 percent of those with multiple sclerosis start with a relapsing-remitting course, in which attacks are followed by partial or total recovery, according to the National Multiple Sclerosis Society. More than half go on to develop a more progressive form of the disease, in which symptoms worsen over time and there are fewer, shorter periods without symptoms. Over time, the disease can lead to loss of vision and paralysis.

The study included data on nearly 2,700 multiple sclerosis patients from British Columbia, Canada, with relapsing-remitting MS who were followed for four to 11 years. About one-third of the patients were treated with interferon beta after it became available in the early 1990s and one-third were not treated with interferon beta. The researchers also examined data on a third group of MS patients who were diagnosed and followed before interferon beta was a treatment option.

The investigators found no statistically significant difference in how long it took for patients prescribed interferon beta to become disabled, defined as needing a cane to walk 330 feet.

"We were not able to find a significant association between interferon beta exposure and progression to disability," said Helen Tremlett, an associate professor of neurology at the University of British Columbia.

Most patients with multiple sclerosis, though not all, develop severe disability 10 to 20 years after diagnosis, according to background information in an accompanying editorial. Several studies have suggested that interferon beta, by reducing relapses, could also prolong progression to disability, Tremlett said. Still other research has found that brain scans of people taking interferon beta show less damage.

Though the exact mechanism of how interferon beta benefits multiple sclerosis patients isn't fully understood, the drug acts on the immune system and reduces inflammation.

"What the field pretty strongly believed is if you reduced relapse rate, surely that should translate into beneficial impact on disease progression," Tremlett said. "Subsequent studies seem to indicate there is disassociation between relapses and long-term irreversible disease progression."

Interferon beta is typically given by injection when people are first diagnosed.

That shouldn't change as a result of these findings, said Timothy Coetzee, chief research officer for the National MS Society. The new study doesn't negate prior studies that have found that the drugs reduce the frequency of relapses.

"If patients are taking the existing disease-modifying therapies [such as interferon beta] they should continue to take them," Coetzee said. "Those drugs were approved for treating MS based on strong clinical trial results that found over a two- to three-year period, people did better with the drugs."

And the ability of interferon beta to stave off disability remains an open question, he added. A recent study by researchers in Italy found that "disease-modifying" drugs such as interferon were associated with a lowered risk of progressing to a more severe form of multiple sclerosis.

What is clear is that multiple sclerosis is a complex disease with many factors involved in how quickly people progress to disability. Interferon beta's action may affect one element of the immune system, but there are other aspects of the immune system and other biological processes at work that over time may overwhelm the drug, Coetzee noted.

"We still have a long way to go to understand what are all the variables that contribute to damage to the brain that leads to the symptoms," Coetzee said. "What we know is there are a lot of players involved in attacking the nervous system of MS patients, that interferon hits some of them, but there are other components of the immune system that get involved."

Coetzee also noted that the researchers did not have data to measure other aspects of living with multiple sclerosis, such as quality of life, thinking skills, vision and other sorts of mobility other than walking.